1. Field of the Invention
The present invention relates generally to optical communication systems. It particularly relates to a capacity-efficient restoration architecture for an optical communication system.
2. Background Art
The operations, administration, maintenance, and provisioning of optical fiber communication systems are described in the Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) standards as specified by American National Standards Institute (ANSI) and International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). SDH is specified in ITU-T G.707 Recommendation, Network node interface for the SDH.
Typical optical fiber communication systems comprise a combination of transmitters, receivers, optical combiners, optical fibers, optical amplifiers, optical connectors, and splitters. Wavelength Division Multiplexing (WDM) or Dense Wavelength Division Multiplexing (DWDM) systems also comprise couplers to enable multiple wavelength transmission over the same optical fiber. Typical optical system configurations include mesh networks and ring networks. Ring networks commonly comprise two fiber pairs connecting a plurality of nodes in a loop. One fiber pair carries bi-directional aggregate traffic between pairs of nodes in the ring. The second fiber pair is used to re-route traffic when there is a failure in the ring on a shared basis. A two-fiber ring is also available in which half the capacity within a fiber is reserved for traffic restoration. Mesh networks commonly comprise a plurality of nodes wherein a node can be connected to more than two nodes in the network enabling enhanced network reliability and higher capacity efficiency when a link failure occurs.
Optical fibers carry far greater amounts of information than carried by other communication media (e.g., electrical cables). Under the Synchronous Optical Network (SONET') standard, the commonly used OC-48 protocol operates at 2.488 Gbps supporting a capacity equivalent to over 32,000 voice circuits. The next highest protocol, OC-192, operates at 9.953 Gbps supporting a capacity equivalent to over 128,000 voice circuits. Therefore, robustness and reliability is required from such high-capacity, long-haul systems. Indeed, most Transatlantic cable systems (TAT), undersea systems which carry international telecommunication traffic, are required to have at least 25 year reliability.
However, since reliability is never absolute most optical systems require a restoration scheme to maintain some level of system performance despite fiber outages, amplifier failures, and some other equipment failure. Several common restoration schemes commercially used include those specified in the SONET standard in a point-to-point single link configuration or a ring network configuration.
Examples of these standardized traditional protection schemes are shown in FIGS. 1, 2. Particularly, FIG. 1 shows a typical one-line point-to-point 1:1 protection system in a Dense Wavelength Division Multiplexing (DWDM) scheme wherein nodes A, B are linked nodes within an optical fiber communication system. The system shown operates in accordance with the SONET/SDH standard, the standard for synchronous data transmission on optical media.
The protection system architecture 100 includes protection switches 110, 190, working and protection link 150, and dense wavelength division multiplexers (DWDMs) 120, 160. Working and protection link 150 commonly comprises a single or multiple (cable bundle of fibers) optical fiber connection between nodes A, B. Protection switches 110, 190 commonly comprise optical-to-electrical transducers and/or optical layer cross-connection switches that provide communication service connectivity between the protection system 100 and other communication devices (e.g., customer premises equipment). There exists a one-to-one correspondence between working channels (lines) 130, 170 and protection channels (lines) 140, 180. However, both working and protection channels 130, 170, 140, 180 are multiplexed by the DWDMs on to a single optical fiber connection between DWDMs 120, 160 for one direction (e.g., A to B). Another corresponding fiber is typically used for the other direction traffic from B to A.
In response to a failure in the transmitter or receiver or cabling for a working line, the SONET/SDH signals carried by working lines 130, 170 are switched from the working lines 130, 170 to the protection lines 140, 180 by protection switches 110, 190. However, since both working lines 130, 170 and protection lines 140, 180 are carried by the same working and protection link 150, a fiber cut in link 150 or a failure in DWDMs 120, 160 or in an optical amplifier for link 150 completely terminates optical communication services between nodes A, B over link 150. To resume service, alternate routing (not shown) would be necessary that can be accomplished through ring switch or mesh restoration means.
FIG. 2 shows the same protection configuration but now with a two-line point-to-point 1:1 protection architecture 200. The protection system architecture 200 includes protection switches 210, 295 working link 250 and protection link 260, and DWDMs 220, 270. DWDMs 220, 270 multiplex working lines 230, 280 and protection lines 240, 290 on to separate working link 250 and protection link 260 between nodes A, B.
For this protection scheme, in response to a failure in the transmitter or receiver or cabling for a working line as well as an optical amplifier or DWDM failure, the SONET/SDH signals carried by working lines 230, 280 are switched from the working lines 230, 280 to the protection lines 240, 290 by protection switches 210, 295. However, again, to resume service when both working and protection links 250, 260 both fail or are cut because the fibers in lines 250 and 260 are in the same cable, alternate routing (not shown) would be necessary that can be accomplished through ring switch or mesh restoration means.
Both 1-line or 2-line 1:1 DWDM systems shown in FIGS. 1, 2 are inefficient in terms of utilization of protection capacity. Both systems use 100% idle capacity that either does not generate any revenue or provides low-grade service on the protection lines. This low-grade service can be preempted when there is a failure of the primary revenue-generating service.
FIGS. 3, 4 again show a commonly-used optical restoration system architecture that provides communication services in accordance with the SONET standard. Particularly, FIG. 3 shows a one-line 1:N protection system using DWDM. The protection system architecture 300 includes protection switches 310, 390 working and protection link 350, and DWDMs 340, 360. Nodes A, B within the system are interconnected by working and protection link 350. In the 1:N protection scheme, there is one dedicated protection channel (line) 330, 380 for each group of N (N>1) working channels (lines) 320, 370. A typical example may be ten groups of 4 (N=4) working channels therein resulting in 10 protection channels for a total number of 40 working channels. In the illustrative example shown in FIG. 2(a), a transmitter/receiver failure on one of a group of N working channels 320, 370 is protected by switching to a protection channel 330, 380 dedicated for that group. Again, due to the one-line scheme for working and protection link 350, an optical amplifier failure or fiber cut results in a termination of communication services between nodes A, B over link 350. Working channels 320, 370 must be re-routed using a ring or mesh restoration network (not shown).
Similarly, FIG. 4 shows a two-line 1:N (N>1) protection system using DWDM. The protection system architecture 400 includes protection switches 410, 495 working link 450 and protection link 460, and DWDMs 440, 470. Nodes A, B within the system are interconnected by working link 450 and protection link 460. For a DWDM or optical amplifier failure, or fiber cut even in a two-line DWDM configuration, (N−1) channels from each group of N working channels 420 will not be restored. Therefore, in our current example assuming ten groups of 4 (N=4) working channels, there are only 10 restoration channels resulting in 30 channels [(N−1)*10] not being restored.
Additionally, even a 2-line 1:N protection using DWDM does not efficiently utilize idle capacity. When the working DWDM link 450 is used to its maximum capacity, only 1/N fraction (e.g., ¼ fraction for current example) of the working channels 420 is used in the protection DWDM link 460 thereby not utilizing the protection DWDM link 460 to its maximum capacity. Therefore, the two-line 1:N protection system inefficiently utilizes the capacity of the protection link although it is more capacity-efficient than the two-line 1:1 protection system which has 100% idle capacity. However, the two-line 1:1 protection system offers better reliability as all working channels in the failed working link will be effectively switched to the idle protection link as contrasted to the 1/N protection capability of the two-line 1:N system.
Another category of restoration schemes include systems which are not confined to a single link. These systems include Bi-directional line switched rings (BLSR) and mesh restoration. These systems have the advantage that the protection capacity is utilized on a shared basis for failures in multiple links within the ring. Particularly, BLSRs typically comprise four fiber rings wherein traffic in one direction travels on one fiber pair while traffic in the opposite direction travels on the other fiber pair. This scheme uses 1:1 configuration for each link of the ring, but the same protection lines of the links of a ring are also used for protection against a fiber cut type of failure when both working and protection lines of another link fail. For failures which affect only the working channel on a route, the signal is protected by using the 1:1 protection scheme previously described. For failures that affect both the working and protection lines of a route, the signal is restored using the protection line carrying traffic traveling around the other direction of the system. The same protection line is used on a shared basis when both working and protection channels fail anywhere within the ring. It would require much more capacity to provide similar reliability as a BLSR against fiber cuts using a 1:1 protection scheme. In the 1:1 scheme, the working line and the protection line between any pair of protection switch systems need to be routed via DWDMs in the opposite directions in the loop.
However, the BLSR system still does not offer the most capacity efficient network when typically there are more than two fiber routes at most of the nodes. This system is limited to particular applications and to only two types of service grades due to the two-fiber architecture. These two grades are the fully-protected service on the working channels in a fiber or the pre-emptable service carried by the protection channels.
A mesh restoration scheme offers some additional advantage by sharing the protection capacity more efficiently than BLSR. Mesh restoration offers 1-line or 2-line 1:1 protection for each link or some mesh restoration architectures use a 1:N protection scheme for each link. The protection channels are also used for restoration against a link failure. Again, 1:1 protection makes inefficient use of protection capacity while 1:N protection offers lower reliability for working channels or poor utilization of DWDM capacity in 2-line 1:N protection configuration.
In view of the above, there is a need to maintain capacity without incurring additional costs when restoring optical communication links. Idle capacity utilization needs to be improved while providing multiple grades of protection (varying priority levels) for different type of communication services. Due to the disadvantages of prior restoration techniques, there is a need to restore communication links and paths while still limiting costs and still maintaining capacity. The present invention describes such a capacity-efficient restoration architecture that dynamically restores failed optical communication links without incurring costs from idle protection links while still maintaining the same capacity or in some instances actually improving capacity from the failed link.